Question

Why did Columbus think he reached Asia

Answer 1

because he had ancesters of african decent who were strung out on drugs, therefore they had no sense of direction

Answer 2

Columbus underestimated the worlds size and calculated that east Asia must be approximately where North America is on right now. They didn't know the Pacific Ocean even existed. He thought he was taking a route to Asia, and thought he had landed in India, which he called the native Indians.

For a shorter answer it was because most of Europe did not know the Americas existed..